Regulatory - Additional Information (Detail) $ in Millions				3 Months Ended			6 Months Ended
	Jun. 29, 2018 USD ($) MW	Dec. 12, 2017 USD ($) MW	Sep. 27, 2017 USD ($) $ / kWac MW	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Oct. 31, 2013 USD ($)
Public Utilities General Disclosures [Line Items]
Storm damage reserve									$ 56
Regulatory assets				$ 392			$ 392	$ 433
Reduction in O&M expense for amortization				5
Tax reform expense (benefits)					$ 102
Regulatory liabilities				1,266			$ 1,266	1,285
PGS [Member]
Public Utilities General Disclosures [Line Items]
Settlement agreement, approval date							Feb. 07, 2017
Regulatory liabilities				5			$ 5
Tax Reform and Storm Settlement [Member]
Public Utilities General Disclosures [Line Items]
O&M expense and regulatory liability					$ 19
Hurricane Irma [Member]
Public Utilities General Disclosures [Line Items]
Storm restoration costs						$ 102
Hurricane Irma [Member] | Storm Reserve [Member]
Public Utilities General Disclosures [Line Items]
Storm restoration costs						90
Hurricane Irma [Member] | O&M Expenses [Member]
Public Utilities General Disclosures [Line Items]
Storm restoration costs						3
Hurricane Irma [Member] | Capital Expenditures [Member]
Public Utilities General Disclosures [Line Items]
Storm restoration costs						$ 9
Solar Project Cost Recovery [Member]
Public Utilities General Disclosures [Line Items]
Settlement agreement, extended terms			four years through December 31, 2021
Settlement agreement, approval date			Nov. 06, 2017
Cost cap of project | $ / kWac			1,500
Solar generation capacity investments			$ 850
Impact of tax reform through reduction in base revenue days			120 days
Solar Project Cost Recovery [Member] | Last Tranche [Member]
Public Utilities General Disclosures [Line Items]
Solar energy capacity | MW			50
Solar Project Cost Recovery [Member] | First Two Tranches [Member]
Public Utilities General Disclosures [Line Items]
Solar energy capacity | MW			400
Maximum cost to be constructed to build last tranche | $ / kWac			1,475
Solar Project Cost Recovery [Member] | Effective September 1, 2018 [Member]
Public Utilities General Disclosures [Line Items]
Solar energy capacity | MW		145
Estimated annual revenue requirements		$ 24
Solar Project Cost Recovery [Member] | Effective January 1, 2019 [Member]
Public Utilities General Disclosures [Line Items]
Solar energy capacity | MW	260
Estimated annual revenue requirements	$ 46
Storm Restoration Cost Recovery [Member]
Public Utilities General Disclosures [Line Items]
Minimum cost recovery period							12 months
Replenishment reserve for recovery of cost									$ 56
Storm Reserve [Member]
Public Utilities General Disclosures [Line Items]
Regulatory assets				9			$ 9	$ 47
Tax Reform and Storm Settlement [Member]
Public Utilities General Disclosures [Line Items]
O&M expense and regulatory asset				$ 35